The present invention relates to an asymmetrical thyristor (ASCR) and to a method for producing such a thyristor. More particularly, the present invention relates to an asymmetrical thyristor composed of a wafer-shaped semiconductor body having at least four zones of alternatingly opposite conductivity types in which the two outer zones are respectively of highly doped n and highly doped p, conductivity types, and the n-conductive base zone is composed of a weakly doped n-conductive layer which is adjacent the inner p-conductive base zone and a higher doped n-conductive layer which is adjacent the outer p.sup.+ conductivity-type layer.
Asymmetrical thyristors whose blocking capability in the reverse direction is relatively small compared to their blocking capability in the forward direction are disclosed in an article by J. T. Fichet et al, "High Current High Voltage Asymmetrical SCRs", PROCEEDINGS OF 29th IEEE ELECTRONIC COMPONENTS CONF., N. Y. 1979, pages 75-79. The principle of such a prior art asymmetrical thyristor is shown in FIG. 1 which is a schematic illustration of an asymmetrical thyristor.
The asymmetrical thyristor (ASCR) of FIG. 1, like a normal thyristor (SCR), is composed of four successive zones of alternatingly opposite conductivity types to provide a pnpn structure, with the two outer zones 1 and 4 being called the emitter zones and the two inner zones 2 and 3 being called the base zones. In a conventional manner, the p.sup.+ -type emitter zone 1 is followed, toward the interior, by the n-type base zone 2 which is followed by the p-conductive control base zone 3 which is adjacent, toward the exterior, to the n.sup.+ -type emitter zone 4 and which conventionally has portions which extend to the outer surface plane of the n.sup.+ -type emitter zone 4, (not shown in FIG. 1).
Between the p.sup.+ -type emitter zone 1 and the n-type base zone 2 there is a pn-junction J.sub.1, between the n-type base zone 2 and the p-type control base zone 3 there is a pn-junction J.sub.2 and between the p-type control base zone 3 and the n.sup.+ -type emitter zone 4 there is a pn-junction J.sub.3. The outer surface of the p.sup.+ -type emitter zone 1 is alloyed or soldered onto a substrate disk 5 which is provided with a terminal 6 to supply current. The outer surface of the n.sup.+ -type emitter zone 4 is provided with a metal contacting layer 7 and a terminal 8 to supply current. In a conventional manner, the exposed surface of the p-type control base zone 3 is provided with a contacting layer (not shown in FIG. 1) and a control terminal. The n-type base zone 2 is at floating potential.
In contrast to a normal thyristor, the n-type base zone 2 of an asymmetrical thyristor is composed of two layers, namely a weakly n-doped or n.sup.- -type layer 2a adjacent the control base zone 3 and a higher n-doped or n-type layer 2b adjacent the p.sup.+ -type emitter zone 1. Due to the relatively high doping concentration of the n-type base layer 2b, the reverse blocking capability produced by pn-junction J.sub.1 is relatively small, for example, about 100V. However, the forward blocking capability produced by pn.sup.- -junction J.sub.2 is high, for example, about 1,300V, because of the low doping and the relatively great thickness or width of the n.sup.- -type base layer 2a.
In such a thyristor, it is desirable to realize the greatest possible and permissible forward off-state voltage V.sub.DRM. That means that the area below the field intensity profile E(x) must be made as large as possible, with the maximum field intensity being less than the critical field intensity Ecrit at which avalanche breakdown occurs. For this reason, prior art devices, as disclosed, for example, in an article by P. Van Iseghem, "p-i-n Epitaxial Structures for High Power Devices", IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. Ed. 23, No. 8, August 1976, pages 823-825, and in FIG. 1 and in FIG. 2 of an article by B. V. Cordingly et al entitled "Design and Performance of an Asymmetrical Thyristor", GEC JOURNAL OF SCIENCE & TECHNOLOGY, Vol, 46, No. 2, 1980, exhibit a "box shaped" field intensity profile or curve in the base zone 2.
According to these prior art devices the field intensity E(x) extends approximately horizontally to or with a very slight slope with respect to the n.sup.- -type layer 2a. In the n-type layer 2b, however, the field intensity E(x) drops relatively steeply to almost zero at the nn.sup.- junction J.sub.hl, so that the space charge zone penetrates only slightly into the n-type layer 2b, which has an advantageous effect on the forward blocking capability. The described field curves are realized by a low doping concentration in the n.sup.- -type layer 2a and a relatively steep rise of the doping concentration from the n.sup.- -type layer 2a to the n-type layer 2b.
In such known asymmetrical thyristors there now occurs the problem that, during commutation from the on-state to the off-state and subsequent reapplication of the positive voltage before the end of the turn-off time, the function of the devices is adversely affected by the inevitable refiring, especially at higher rates of rise of the forward off-state voltage. The adverse effect is that after every commutation, the reverse voltage V.sub.RRM of the thyristors continues to decrease, and finally the thyristors may be destroyed completely which is a severe drawback for the practical use of such thyristors.